Optical networks are usually organized in ring or meshed structures consisting of several nodes connected by unidirectional or, more often, bidirectional links. The usage of reconfigurable optical cross-connects introduces the possibility to reroute traffic and reallocate network resources in a dynamic way.
Modern optical transport networks for telecommunications rely upon coherent technology to convey information in the amplitude, phase and polarization of light. Whereas the first generation of optical coherent systems usually employed dual polarization QPSK (DP-QPSK) with hard-decision (HD) forward error correction (FEC), state-of-the-art systems support a variety of mQAM modulation formats and coding schemes, where m could e.g. be 16, 32, or 64. In particular, multi-rate multi-format transponders allow to choose coding and modulation according to the characteristics of the link at hand.
Transport networks are critical telecommunication infrastructures and are therefore subject to strict reliability constraints. An important requirement, often referred to as “survivability”, is the ability of the network to provide the committed quality of service (QoS) in several failure scenarios, as regulated by the service level agreement (SLA) between network customers and provider. Although failure protection mechanisms can be realized in different layers of the protocol stack, in this disclosure particular focus is put on implementations at the physical layer.
Generally, whenever a link failure is detected, the traffic is rerouted onto an available backup link. In some cases a dedicated backup is assigned to some critical links, but typically a shared backup offers a better trade-off between reliability and costs. According to this approach, the network is provided with a certain amount of over-capacity, which is used in the case of failure to establish the required protection paths. At least for single failure scenarios on the most critical links, the protection paths are often preplanned to guarantee the shortest possible service interruption. Note that in the present disclosure, the terms “path” and “link” are used interchangeably.
In case of link failure, the preplanned or calculated protection link at the optical transport layer might possibly support only a fraction of the bit rate supported by the working link. This happens for instance when the protection path is impaired by stronger noise, nonlinear fiber effects, and/or filtering effects than the designated working path or “given link”. This situation is not uncommon, especially when the backup is shared and hence cannot be finely optimized as the working path. However, depending on the SLA, a reduced throughput over the protection link in some fault scenarios is an acceptable option. In this case higher layers must take care of recovering the remaining traffic.
The flexible nature of multi-rate multi-format transponders offers a convenient way to down-grade the end-to-end throughput, by adapting the coding and modulation schemes. For instance, a 200 Gb/s signal transmitted over the working link using DP-16QAM could be downgraded to a 100 Gb/s signal using DP-QPSK to maintain satisfactory transmission quality over a longer protection link, by discarding the low-priority traffic. The adaptation of the transmission format could either be requested by a central control unit or could be negotiated autonomously between the involved transponders. Unfortunately, both approaches require, besides some signaling overhead, a full reconfiguration of the transponders at the end nodes of the link, which would typically include reprogramming registers of framer and modem devices, tuning analog oscillators and adjusting analog signal levels. This process is, however, extremely time-consuming and results in practice into severe service interruption which will often not be tolerable for high-priority traffic.
If the long reconfiguration time of the transponders and the additional signaling overhead shall be avoided, the problem could be circumvented, rather than solved, by constraining all protection paths to support at least the same throughput as their respective working path. However, this solution comes at a tremendous cost in terms of required overcapacity in the network. Further, this cost increases with the network size due to the growing number of possible failure scenarios that must be taken into account.
As an alternative, the throughput on the working path could be artificially lowered below the actual link capacity according to the transmission conditions of the worst-case protection path. Obviously, also this solution comes at a high price, because it sacrifices throughput during normal operation to guarantee a quick failure response.